Donors to political parties are securing "preferential" access to top decision makers, creating the risk that policy is being influenced in "subtle" ways, the head of the standards watchdog has warned.

Sir Christopher Kelly, chairman of the committee on standards in public life, said people were right to be suspicious of the motives of anyone who donates more than £10,000 to a party, and repeated the call he made in November for donations to be capped at that level annually.

"There have been sufficient allegations in the past of people receiving honours, or policies being influenced, for people to think there is no smoke without fire," he told the Daily Telegraph.

Kelly, a former civil servant who was appointed to his current post in 2007, said that there was "no doubt" that significant donors have preferential access to political decision makers.

"The thought that anyone would give such a large sum of money to a party solely for altruistic reasons is quite a difficult one," he added.

"The risk is policy being influenced in other, more subtle, ways because some people have access because they have given donations.

"There is a risk of it [influencing of policy by donors] happening and more importantly there is a public perception that it does happen."

He also said that state funding to political parties should be increased to remove the potential taint of sleaze. His committee has proposed that from 2015 there should be a £3-per-vote state funding for the parties, or £23m a year over five years.

It has also suggested that trade union political levy payers be required to state in writing that they wish to affiliate to the party, sanctioning the donation of £3 per individual to the party from the union's political fund.

Kelly warned in November that another funding scandal is inevitable unless the political parties show some leadership and back radical changes, including the £10,000 annual cap.

A poll carried out last year by the standards watchdog found that eight out of 10 members of the public thought the wealthy donated more than £100,000 only in the hope of receiving special favours.